Electrical apparatus having insulation for eliminating creepage tracking



Aug. 22, 1961 ss ET AL 2,997,528

ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING CREEPAGE TRACKINGOriginal Filed Jan. 9, 1957 2 Sheets-Sheet 1 lnveniors: Alvcn A.KeSSelROberT S. N rman Thelr Airorney Aug. 22, 1961 A. A. KESSEL ET AL2,997,528

ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING CREEPAGE TRACKINGOriginal Filed Jan. 9, 1957 2 Sheets-Sheet 2 lnventors Aivcm A. KesselRoberi S. Norman by wu'dg/ Their AHorney Patented Aug. 22, 19612,997,528 ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING(CREEPAGE TRACKING alvan A. Kessel, Wakefield, and Robert S. Norman,

Marblehead, Mass assignors to General Electric Company, a corporation ofNew York )riginal application Jan. 9, 1957, Ser. No. 633,356. Dividedand this application Feb. 14, 1958, Ser. No.

6 Claims. 01. 1'74 137 The present invention relates to improvements inthe nsulation of electrical apparatus and, more particularly, 0electrical devices and improved insulations therefor vhich are subjectto contaminated conditions which 'romote creepage electrical dischargeconditions. As s well known, certain types of electrical equipmentsubacted to contaminating atmospheric conditions such as moisture, dust,fog and salt frequently fail due to reepage between points of differentpotentials on the quipment. While the insulating components of elecricalequipment desirably include organic materials vhich are inexpensive andreadily molded or otherwise abricated, it has been a particulardisadvantage that he organic constituents of such components tend to ormcarbonaceous deposits upon exposure to conditions vhich promote suchcreepage. These carbonaceous devosits ultimately provide paths ofsufficiently low reistance to occasion breakdown of the equipment. This,pplication is a division of our copending application,

ierial No. 633,356, filed January 9, 1957 which latter .pplication is inturn a continuation-in-part of our coending application, Serial No.523,445, filed July 21, 955 now abandoned.

By way of example, in the instrument transformer ield, it has heretoforebeen a distinct limitation to the lsefulness of dry molded instrumenttransformers that he molded organic insulation components thereof formow-resistance carbonaceous deposits upon exposure to reepage electricaldischarges. High-voltage transformrs of this dry type may include moldedorganic inulating compounds not only as the insulation between lementsbut as the outer protective casing as well, and re thus distinguishedfrom those which are filled with il or other insulating liquids. Inoutdoor installations, '1' others where there may be accumulations ofdust, rain 11d other environmental contaminants, random surfacelischarges or arcing known as surface creepage are |romoted betweenelements. These discharge conditions ICCQSIOI]. the formation ofcarbonaceous deposits in the usulation, ultimately yieldinglow-resistance paths or racks which destroy further utility of theapparatus. )ischarges of the creepage type are to be distinguished romthose caused by the establishment of an are through r directly betweentwo parts of the apparatus having lilferent potentials. Under arcingconditions, while the vrganic material adjacent to the arc iscarbonized, the re track so formed is not random in character but formsdirect path along the line of the arc. On the other and, tracks due tocreepage are random in effect and roduce a tree-like path. Thedifference between trackng due to arcing and tracking due to creepage isfurther lointed out in ASTM Test D495-48T in which it is tatedspecifically that the test directed to determining he resistance ofinsulating material to arcs does not in ;enera1 permit conclusions to bedrawn as to the reistance of the material to other types of arc such ashose promoted by conducting contaminates with which atter the presentinvention is concerned. It is further vointed out that in the creepagetype of electrical failure, ailure of the material can occur not onlydue to surace failure but to subsurface failure. In other words,

even if the surface of the organic material is devoid of carbonaceous orconducting material, tracking due to surface creepage may neverthelessoccur between two points of different potential beneath the surface ofthe material itself. It is evident from the above and it has been foundthat materials which are effective in protecting against the effects ofdirect arcing are not necessarily effective in protecting againstcreepage breakdown.

As a result of such disadvantages, organic insulating components havebeen avoided in the construction of electrical equipment wherein suchcomponents would be subjected to the influences of creepage electricaldischarges, even though these components would otherwise have beenattractive because of different considerations. Organic insulatingmaterials which experience this limitation include polyester resins.

Accordingly, it is one object of this invention to provide improvedelectrical apparatus having polyester resin organic insulation whicheliminates tracking due to creepage electrical discharge conditions.

It is another object to provide improved electrical apparatus includingcertain polyester resin compositions which have insulatingcharacteristics and in which formation of carbonaceous deposits uponexposure to creepage type electrical discharges is eliminated.

By way of a summary account of one practice of the teachings of thisinvention, an improved high-voltage outdoor instrument transformerincludes a magnetic core member with linked primary and secondarywindings each connected to terminals, and a polyester resin compositionhaving hydrated alumina dispersed therein in proportions definedhereinafter. The polyester composition with the hydrated alumina isformed about the transformer elements to electrically insulate them toprovide an outer covering which protects these elements againstinfluences of the ambient environment. Uniquely, the creepage dischargesoccurring across the surface of the transformer even under the mostsevere contaminating conditions do not occasion tracking and breakdown,inasmuch as low-resistance carbonaceous deposits are not permitted toform.

Although the features of this invention which are novel are set forth inthe appended claims, greater detail of the invention in its preferredembodiments and the further objects and advantages thereof may bereadily comprehended through references to the following descriptiontaken in connection with the accompanying drawings, wherein:

FIGURE 1 provides a perspective view of a typical molded casing typetransformer embodying our invention;

FIGURE 2 is a cross-sectional view of the transformer of FIGURE 1; and

FIGURE 3 is a pictorial view of electrical test equipment utilized toprocure operating life data.

Referring to FIGURES 1 and 2 of the drawings, the electrical apparatusthere shown is a current transformer having primary terminals 19 and 11and secondary terminals 12 and 13. The positioning of the primarywindings l4 and the secondary windings 15 will be apparent from the viewof FIGURE 2. These windings are electromagnetically linked with a core16 of magnetic material which is provided with a pair of tubularseparators 17 and 18 which serve to insulate the core from the windings.

The transformer of FIGURES l and 2 includes composition 19 whichencapsulates the various components as shown and thereby provides acompleted physical enclosure as Well as electrical insulation. A supportplate 20 facilitates mounting. As has been pointed out above, organicinsulating materials heretofore employed as such components oftransformers of this type, and in other applications wherein similarconditions may occur, have been subject to the basic problem ofcarbonaceous residue formation and accumulation occasioned by randomelectrical creepage discharges which are likely to occur underadverseclimatic conditions in outdoor installations. This difficulty isattributable to the inherent characteristics of organic materials ofthis type which cause them to break down to form low-resistancecarbonaceous deposits when exposed to creepage type electricaldischarges. Composition.19 includes polyester resin and hydrated aluminadispersed therein in sufficient quantity to prevent accumulation ofcarbonaceous deposits upon exposure to creepage electrical dischargessuch as those which may occur between the conducting members 10, ll, 12,13 and 20.

With apparatus not embodying our teachings, it is believed that thebreakdown process with respect to creepage failures occurs as follows.The localized high-temperature discharges produced across the surface ofthe polyester resin breaks down the polyester resin components to formcarbon deposits. The process is cumulative, as has been previouslydescribed, and low-resistance tracking failure occurs rapidly.

Electrical apparatus wherein exposure to contaminating conditions islikely to occur and in which our teachings may be embodied to greatadvantage includes also apparatus other than current transformers whichare unprotected from such contaminating conditions. In accordance withone practice of our invention, we provide highvoltage electricalapparatus in which conductors having a large potential difference arefound with an insulating polyester resin material having interspersedtherein a hydrated alumina in proportions such that material breakdownsordinarily experienced under creepage electrical discharge conditions donot occur. The concentration of the hydrated alumina employed iscritical since it has been found that only a sufficient concentrationeliminates carbon accumulation and electrical breakdown. As will bedemonstrated, this effect can be achieved by employing a hydratedalumina only in the hereinafter defined critical proportions, A verymarked increase in operating life of the apparatus under creepageelectrical discharge conditions is observed as the critical levels ofconcentration are reached.

Using polyester resins the hydrated almina is used in the proportion offrom to 70% by weight of the insulation and preferably to 70% Thepreferred specific hydrated alumina content of each insulatingcomposition is 60% by weight.

While we do not wish to be bound by any particular theory, it beingsufiicient that our invention accomplishes the desired end, we believethat the combined water in the hydrated alumina serves to oxidize thecarbonaceous particles formed under creepage conditions and that thealuminum oxide component itself acts as a catalytic agent to indirectlypromote this oxidation. It has been found that while unhydrated aluminumoxide is useful in delaying tracking under surface creepage conditions,it is not efficacious in oxidizing carbonaceous materials or inpreventing the eventual failure of insulation due to creepage tracking.It has also been found that the water must be chemically bound to thealuminum oxide as in the hydrated compound. It will be obvious that inpractical commercial applications, failure due to surface creepagetracking must be not merely delayed but wholly eliminated as by thepractice of our invention.

In order that those skilled in the art may better understand how thepresent invention may be practiced, the following examples are given toillustrate organic insulation components containing polyester resinmaterials coming within the scope of this invention. These examples aregiven by way of illustration and not by way of limitation.

Example I Tests were conducted with apparatus mock-ups including variousinsulating compositions of polyester resin and selected.concentrationsby.weight of hydrated alumina of F trodes extendingparallel to each other.

the chemical composition Al O .3H 0. These samples of insulationcomponents were prepared as molded sheets about 0.076" thick and about6" square and were mold cured at 150 for about 20 minutes. For testingpurposes these samples identified by numeral 21 in FIG. 3 were mountedin a high voltage test cage resting flat against a conducting metalplate 22 tilted at an angle of about 15 from the horizontal.

Two electrodes, 23 and 24, each about 1" by 2" in cross section wereplaced perpendicularly against the surface of each sample 21 under test,that is, against the upper side of the sample opposite that in contactwith the metal plate 22. The electrodes '23 and 24 were positioned about1" apart with the 2" dimensions of the elec- One of the two electrodes,identified by numeral 23, was connected to the metal plate 22 by clips25 and 26 and conductor 27. The two electrodes were electricallyconnected across the alternating voltage output of an adjustablehigh-voltage transformer through clips 28 and 29. It will be appreciatedthat, with the circuit just described, creepage breakdown of the samplemay occur between the two electrodes 23 and 24 or between electrode 24and the metal'plate 22 by means of a surface failure, or betweenelectrode 24 and the metal plate 22 through the thickness of the sampleby means of an erosion type failure.

After the test samples were placed in the apparatus as just described,they were dusted with a synthetic dust representative of atmosphericdust accumulations, the dust particles being designated by referencecharacter 30. A fine water spray 31 was directed against the samples forthe duration of the test, a nozzle 32 being coupled with water and airlines 33 and 34 for this purpose. A 60-cycle voltage of about 1500 voltswas applied to the electrodes 23 and 24 to set up surface dischargeconditions of the intensity capable of causing decomposition of organicmaterials. The samples were removed from the apparatus for redustingpurposes at the end of each hours of testing, or sooner for inspectionpurposes if failure occurred.

It will be observed by referring to the dust-spray test data presented,that a large increase in operating life of the apparatus underelectrical discharge conditions is obtained as the concentration ofhydrated alumina is increased beyond a certain level. A more significantconsideration is, however, the concentration level at which the type offailure encountered changes from a surface type failure to an erosiontype failure.

The surface failure is one which occurs by reason of formation of arandom carbon path along the surface of the material indicating that thelife of the material is being limited by carbonization, while an erosionfailure is one which occurs because the insulation component 21 isreduced in thickness by an eroding effect not involving carbonization.That is, it is determined by the inherent qualities of the insulationcomponent itself rather than by some external cause such ascarbonization.

Hence, at the point where erosion failure begins to occur instead ofsurface failure, the concentration of hydrated alumina has reached alevel where carbonization is no longer the limiting factor with respectto ability of the high-voltage apparatus to withstand creepageelectrical discharges. The samples used to obtain the data presentedwere inspected upon failure to determine the type of failure involved ineach case; that is, whether breakdown occurred across the surfacethrough carbonization or whether it occurred through the thicknessthereof by erosion between the ungrounded electrode 24 and the metalplate 22.

Test pieces as described above were made of commercially availablepolyester resin known as Laminac No. 4116 manufactured by AmericanCyanamid Company to which was added 1% by weight of tertiary Butylperbenzoate, as a catalyst, based on the weight of the resin.

When this material with added hydrated alumina was tested as above, theresults were as follows:

Example I was repeated except that the polyester used was H. H.Robertson Companys Stypol 107E catalyzed with 0.5 part of benzoylperoxide per each 100 parts by weight of resin. When hydrated aluminawas mixed with the resin in varying amounts and the material tested asin Example I, the results were as follows:

TABLE II Material Percent Hours to Failure Type of A12Oa-3H2O Failure 01 Surface. 50 SuEriace and H. H Robertson T951011 20 180 c. EIOSlOll.stypol 107E 30 Greater than 200---. Do. 40 .do Do. 50 .do D0.

From Examples I and II it will be seen that polyesters mixed withhydrated alumina exhibit desirable creepage resistance. Under dust-spraycondition creep tracking is prevented when from 20 to 70% by Weight ofthe composite insulation is hydrated alumina. Under salt fog conditionsa minimum of about 30% hydrated alumina is preferred to eliminate creeptracking. The upper limit of hydrated alumina is imposed only by thephysical re quirements of the insulation.

The term polyster resin refers to a material comprising the reactionproduct of a polyhydric saturated or unsaturated alcohol and a saturatedor unsaturated polybasic acid either with or without a modifyingunsaturated monomer such as styrene, etc. Specific examples of the basicmaterial are, for instance, diethylene glycol maleatc, dipropyleneglycol maleate, diethylene glycol fumarate and so forth. Such materialsare readily polymerized by peroxy catalysts such as benzoyl peroxide,tertiary Butyl perbenzoate, etc.

It will also be apparent that the particular examples set forth above byway of illustration are not limiting in nature and that various forms ofelectrical apparatus having greatly improved resistance to breakdownscaused by creepage electrical discharges may include insulationcomponents having the organic insulating materials set forth above and ahydrated alumina in concentration defined by the foregoing disclosure,and that such arrangements deriving the advantages of our invention,will fall within the true scope and spirit of our invention as definedin the appended claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. Electrical apparatus comprising at least two spaced electricallyconducting members between which electrical potentials may be developed,an organic insulating material disposed and completely filling the spacebetween said members and having an outer surface intermediate saidmembers exposed to ambient contaminating atmospheric conditions, saidsurface material comprising polyester resin which tends to form lowresistance carbonaceous deposits under the influence of creepage typeelectrical discharges occurring under contaminating conditions and meansfor preventing the formation of said carbonaceous deposits due tocreepage, said means comprising hydrated alumina interspersed in saidsurface material in the amount of from 20 to 70 percent based on theweight of said surface material and hydrated alumina.

2. Molded electrical apparatus comprising at least two electricallyconducting members between which electric potentials may be developedand an insulating composition disposed between said members and havingan outer surface intermediate said members exposed to ambientcontaminating atmospheric conditions and fixing said members in spacedrelationship such that electrical discharges of the creepage type mayoccur therebetween and across said surface of said insulatingcomposition, said composition including polyester resin which leaves alow resistance carbonaceous residue under influence of creepage typeelectrical discharges and further including hydrated aluminainterspersed in said composition in the amount of from 20 to 70 percentbased on the weight of said composition and hydrated alumina, saidhydrated alumina preventing accumulation of carbonaceous material uponexposure of said surface of said insulating composition and hydratedalumina to creepage type electrical discharges occurring across saidsurface under contaminating conditions.

3. An electrical insulator shaped to support and hold in insulatingrelationship a plurality of conducting members which are adapted to haveelectrical potentials developed thereacross and between which electricaldischarges of the creepage type may occur, said insulator comprising anorganic insulating material disposed between said members and having anouter surface intermediate said members exposed to ambient contaminatingatmospheric conditions, said surface material comprising polyester resinwhich tends to form low resistance carbonaceous deposits under influenceof creepage type elec trical discharges and hydrated alumina dispersedin said surface material and comprising from about 20 to 70 percent byweight of the combined surface material and hydrated alumina, saidhydrated alumina serving to prevent the formation of said carbonaceousdeposits under said contaminating conditions.

4. Electrical apparatus comprising at least two spaced electricallyconducting members between which electrical potentials may be developed,an organic insulating material disposed and completely filling the spacebetween said members and having an outer surface intermediate saidmembers exposed to ambient contaminating atmospheric conditions, saidsurface material comprising polyester resin which tends to form lowresistance carbonaceous deposits under the influence of creepage typeelectrical discharges occurring under contaminating conditions and meansfor preventing the formation of said carbonaceous deposits due tocreepage, said means comprising hydrated alumina interspersed in saidsurface material in the amount of 60 percent based on the weight of saidsurface material and hydrated alumina.

5. Electrical apparatus comprising at least two spaced electricallyconducting members between which electrical potentials may be developed,an organic insulating material disposed and completely filling the spacebetween said members and having an outer surface intermediate saidmembers exposed to ambient contaminating atmospheric conditions, saidsurface material comprising polyester resin which tends to form lowresistance carbonaceous deposits under the influence of creepage typeelectrical discharges occurring under contaminating conditions and meansfor preventing the formation of said carbonaceous deposits due tocreepage, said means comprising hydrated alumina interspersed in saidsurface material in the amount of from 30 to 70 percent based on theWeight of said surface material and hydrated alumina.

6. In a molded electrical transformer adapted to be energized by asource of electrical power and having spaced high voltage terminals andlow voltage terminals, an insulating material molded about saidtransformer and 7 fixedly positioning said terminals, said materialbeing disposed and completely filling the space between said terminalsand having an outer surface intermediate said'high and low voltageterminals exposed to ambient contaminating atmospheric conditions, theimprovement which comprises at least the outer surface of said materialin cluding polyester resin which leaves a conducting carbonaceousdeposit under the influence of creepage electrical discharges andhydrated alumina dispersed in at least said outer surface of saidmaterial to comprise from 20 to 70 percent by Weight of at least saidouter surface of the combined insulating material and said hydratedalumina, said hydrated alumina obviating said carbonaceous deposits.

References Cited in the file of this patent UNITED STATES PATENTS2,618,689 Cook Nov. 18, 1952 2,645,626 Nordlander et a1. July 14, 19532,679,493 Anderson May 25, 1954 2,743,308 Bardsley Apr. 24, 19562,768,264 Jones et a1 Oct. 23, 1956

1. ELECTRICAL APPARATUS COMPRISING AT LEAST TWO SPACED ELECTRICALLYCONDUCTING MEMBERS BETWEEN WHICH ELECTRICAL POTENTIALS MAY BE DEVELOPED,AN ORGANIC INSULATING MATERIAL DISPOSED AND COMPLETELY FILLING THE SPACEBETWEEN SAID MEMBERS AND HAVING AN OUTER SURFACE INTERMEDIATE SAIDMEMBERS EXPOSED TO AMBIENT CONTAMINATING ATMOSPHERIC CONDITIONS, SAIDSURFACE MATERIAL COMPRISING POLYESTER RESIN WHICH TENDS TO FORM LOWRESISTANCE CARBONACEOUS DEPOSITS UNDER THE INFLUENCE OF CREEPAGE TYPEELECTRICAL DISCHARGES OCCURRING UNDER CONTAMINATING CONDITIONS AND MEANSFOR PREVENTING THE FORMATION OF SAID CARBONACEOUS